The present invention relates generally to heating, ventilating, and air conditioning (HVAC) controls systems and, more particularly, to a system and method for compensating for thermostat delays in activating HVAC systems.
Electric utility companies need to generate enough power to supply the various loads currently demanding power. Traditionally, utilities meet the load demand using automated generation control. The load demand may rise and fall dramatically throughout the day, and the overall load demand generally rises every year. As loads are added to utility grids and demand rises, utilities increase the output of existing generators to meet the increased demand. To solve the issue of continuing long-term demand, utilities typically invest in additional generators and plants to match rising demand. As load levels fall, utilities may reduce generator output or even take generators off line to match falling demand. As the overall demand for electricity grows, the cost to add power plants and generation equipment that serve only to fill peak demand becomes extremely costly.
In response to the high cost of peaking plants, electric utility companies have developed solutions and incentives aimed at reducing both commercial and residential demand for electricity. In the case of office buildings, factories and other commercial buildings having relatively large-scale individual loads, utilities offer differential electricity rates to consumers who install locally-controlled load-management systems that reduce on-site demand. Reduction of any individual large scale loads by such load-management systems may significantly impact overall demand on its connected grid. In the case of individual residences having relatively small-scale electrical loads, utilities offer pricing incentives to consumers who install demand-response (DR) technology at their residences. The DR technology controls high-usage appliances such as, for example, air-conditioning (AC) compressors, water heaters, pool heaters, and so on. Such technology aids the utilities in easing demand during sustained periods of peak usage.
Traditional DR technology used to manage thermostatically-controlled loads such as AC compressors typically consists of a DR thermostat or a load control relay (LCR). Such DR thermostats, LCRs, and other known DR devices are designed to be used with a wide variety of ducted, thermostatically-controlled HVAC systems such as, for example, those commonly used in single-family residences in the United States. Typical ducted HVAC systems in the United States utilize distinct and separate thermostatic devices, circulation fan controls, electrical contactors, switches, and so on that are easily accessible for connection to DR devices.
When an LCR is selected as the DR device, the LCR is typically added to an existing HVAC system. The LCR is often wired into the HVAC system to control the R wire (power wire) to a thermostat, interrupting power to the thermostat when a load or loads of an HVAC system are to be controlled. Interrupting power to the thermostat is often referred to as shedding the load or shedding the LCR and occurs when the LCR is in an open position or state. In contrast, closing the LCR to supply power back to the thermostat is often referred to as restoring the LCR, as power is restored to the thermostat once the LCR is closed. The LCR is typically controlled according to a cycle or shed percentage indicating the percentage of time in a cycle that the LCR should be shed or according to a restore percentage indicating the percentage of time in a cycle that the LCR should be restored.
Many thermostats include an integrated protection timer that delays providing power to the HVAC system loads for a certain amount of time after the LCR has restored power to the thermostat. For example, once an LCR has restored power to a thermostat that is configured to supply power to a Y wire (a call for cool line or cool control line) running to an air conditioner, the thermostat will not supply power to the Y wire until a protection timer has expired. The thermostat or compressor protection delay implemented by the protection timer may be configurable (different from home owner to home owner). But, most often, the protection timer uses a randomized thermostat protection delay so that, if power is restored after a power failure, a utility grid will not experience the combined demand of all the loads that come back online after power has been restored instantaneously.
While the thermostat protection timers provide certain benefits to the utility, property owners experience periods of load shed that are longer than required to receive the lower prices offered by the utilities under the DR system, and those extended periods of load shed cause those property owners to experience greater discomfort. More specifically, the LCR controls are configured assuming that the thermostat provides power to its loads immediately after the LCR is closed. However, the thermostat will not allow any loads to receive power during the thermostat protection delay period, even though the LCR is closed. Thus, from a property owner's perspective, the period when the LCR is restored and the thermostat will not relay power to its load(s) is wasted control time.
It would therefore be desirable to provide a system and method for compensating for the thermostat protection delay time in order to maximize the control period when the LCR is restored.